Extraction Equilibria of Picolinic Acid with Trialkylamine - American

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Extraction Equilibria of Picolinic Acid with Trialkylamine/n-Octanol Ling Zhang, Fei Yu, Zhixian Chang, Yuanyuan Guo, and Deliang Li* Institute of Environmental and Analytical Sciences, College of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, P. R. China ABSTRACT: The extraction equilibrium of picolinic acid using trialkylamine (N235) dissolved in n-octanol at 298 ± 0.5 K has been studied. The factors that affected distribution coefficients (D), such as aqueous equilibrium pH (pHeq), N235 concentration, and diluents, were investigated. As diluents, n-octanol showed higher D values than those of tetrachloromethane and kerosene. The maximum D values appeared at a N235 concentration of 0.4218 mol·L−1, which were in agreement with the results of the apparent alkalinity of N235/n-octanol. The D values were also observed with peak values at pHeq in the range of 4.5−6.0. In addition, D decreased with the increase of the initial picolinic acid concentration. Fourier transform infrared spectrometry confirmed that pHeq had no influence on the complexes’ structure and the extraction reaction belonged to the proton-transfer process. The expression of D was proposed based on the mass action law with reasonable assumptions, and the model parameters were calculated by fitting the experimental data. The D values obtained by the model were in good agreement with the experimental ones.

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INTRODUCTION Picolinic acid (pyridine-2-carboxylic acid), which can be used to produce mepivacaine, is an important intermediate for organic synthesis.1 Picolinic acid is also commonly used as a ligand with metal for wastewater treatment, i.e., Lee2 reported that the increase of picolinic acid concentration could enhance the removal of copper(II)−picolinic acid complexes by active carbon fibers. Every year, large amounts of wastewaters containing picolinic acid are discharged into rivers and leads to serious environmental pollution. Picolinic acid in these kinds of wastewaters has high recyclable values. However, the recovery, especially the separation, of picolinic acid has been rarely reported. Reactive extraction, which is based upon the reversible reaction of the specific functional groups of solute and extractant, is an effective separation technique for organic compounds with high selectivity. It has received increasing attention for recoveries of polar organics from their dilute solutions.3−7 Long-chain aliphatic tertiary amines dissolved in suitable diluents have been considered as efficient extractants for the recoveries of carboxylic acids. Picolinic acid is a typical carboxylic acid compound, thus it is thought desirable to be extracted by amine extractants. Recently, researchers have performed detailed experimental studies on the extractions of the isomers of picolinic acid like nicotinic acid (pyridine-3-carboxylic acid) and iso-nicotinic acid (pyridine-4-carboxylic acid) with various extractants and gained a series of encouraging results. Senol8,9 investigated the extraction equilibria of nicotinic acid using Alamine 336 (or 300) in various conventional diluents (proton donating and accepting, polar and nonpolar diluents). The results showed that conventional solvents were not suitable separation agents for nicotinic acid, yielding D0 ≤ 1, and there was synergistic © 2012 American Chemical Society

extraction efficiency in the amine/cyclic alcohol (such as cyclopentyl alcohol and methyl cyclohexanol) systems. Kumar10,11 aimed to intensify the recovery of nicotinic acid using reactive extraction with trin-octylphosphine oxide (TOPO) or tri-n-butylphosphate (TBP) in different diluents. His work provided lots of equilibrium data for separating nicotinic acid. Caşcaval12 carried out a comparative study on reactive extraction of nicotinic acid using lauryl-trialkylmethylamine (Amberlite LA-2) and di(2-ethylhexyl)phosphoric acid (D2EHPA) and found that Amberlite LA-2 allowed the possibility to reach a higher extraction efficiency compared with D2EHPA. Li13 discussed the distribution of iso-nicotinic acid between water and Alamine 336 dissolved in n-octanol and kerosene. By infrared spectrometry analysis, he deduced the interaction between Alamine 336 and iso-nicotinic acid was a proton-transfer process and calculated the apparent extraction equilibrium constants and complexation ratios. The distribution behavior of picolinic acid between water and Alamine 336 dissolved in various diluents has been studied by Senol.14 It was reported that the distribution degree of picolinic acid in pure diluent was very low (D0 ≤ 1) and the amine/ alcohol always possessed a synergistic extraction effect. However, the effects of the aqueous solutions’ pH, amine concentration, and interaction mechanism of the amine extractant with picolinic acid have not been completely discussed. Herein, systematic works on the extraction of picolinic acid with a kind of amine extractant N235 dissolved in n-octanol were performed. The influences of aqueous equilibrium pH (pHeq), N235 concentration, and initial Received: October 26, 2011 Accepted: January 9, 2012 Published: January 24, 2012 577

dx.doi.org/10.1021/je201314m | J. Chem. Eng.Data 2012, 57, 577−581

Journal of Chemical & Engineering Data

Article

concentration of picolinic acid on distribution coefficients (D) were discussed. The organic phases loading picolinic acid were characterized by infrared spectroscopy to investigate the interaction mechanism between N235 and picolinic acid. Furthermore, an expression of D, used for estimating extraction efficiency, was proposed and the related model parameters were calculated.

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EXPERIMENTAL SECTION Materials. Trialkylamine (N235), a kind of C 8−C 10 saturated straight-chain tertiary amine mixture, was obtained from Rongcheng Petrochemical Plant (Shandong, China). It is a pale yellow liquid practically insoluble in water ( 7.0. The highest D values appeared at a pHeq of 4.5−6.0 and indicated N235 mainly reacted with the neutral and anionic picolinic acids in the extraction process. The initial concentration of solute was also of influence on reactive extraction equilibrium. Picolinic acid solutions with different initial concentrations were examined by extraction with 0.4218 mol·L−1 N235/n-octanol, and the D values were shown in Figure 4. As seen, D values decrease gradually with

Figure 5. D vs pHeq in various 0.6327 mol·L−1 extractants in n-octanol. ■, N235; ▲, P507; and ●, TBP.

capacities of TBP and D2EHPA in n-octanol are rather poor with D < 1.0, while N235/n-octanol demonstrates very good separation ability for picolinic acid. The results further confirmed N235/n-octanol was suitable for separating picolinic acid. Infrared Spectra Analysis. Infrared spectra of N235/noctanol loading with picolinic acid were recorded on an infrared spectrometer as well as the original N235/n-octanol and pure picolinic acid. Figure 6 shows the IR spectra of N235/n-octanol

Figure 6. IR spectra of loading organic phases at pHeq: 1, 3.87; 2,6.53; and 3, 8.88.

loading with picolinic acid at different pHeq values. It shows no significant difference, indicating that pH had an ignorable influence on the complexes’ structure, namely, pH did not affect the interaction forms between N235 and picolinic acid. Figure 7 shows the comparison of IR spectra of N235/noctanol loading with picolinic acid, original N235/n-octanol and pure picolinic acid. As seen, the band at 1720 cm−1 in picolinic acid, assigned to the characteristic stretching vibration of CO, was not observed in the N235/n-octanol loading phase, while two new bands appeared at 1640 and 1557 cm−1, which could be assigned to the characteristic asymmetry and symmetry stretching of COO−. It indicated that the ion-pair complex has been formed between N235 and picolinic acid; in

Figure 4. D vs initial concentrations of picolinic acid.

the increase of initial concentrations of picolinic acid. This might be attributed to that the gradual increase of picolinic acid concentration made the extractant’s loading solute close to stoichiomentric saturation. The amount of extracted solute increased but extraction efficiency decreased. By taking into account the principle of chemical balance motion, this phenomenon could be easily explained. The result indicated 579



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K1 and K2 represent the apparent extraction equilibrium constants of N235 reacting with neutral picolinic acid and protonated N235 reacting with anion picolinic acid, respectively. It is obvious that

[R3NH+][A −] K1 = [R3N][HA] = K2KH

[H+][A −] [HA]

= K2KHK a2 So, the distribution coefficient, D, can be written as Figure 7. IR spectra of 1-picolinic acid, 2-N235/n-octanol, and 3picolinic acid + N235/n-octanol.

⎛ ⎞ K1B0 D=⎜ + vm⎟ + ⎝ KH[H ] + 1 + K1[HA] ⎠

other words, the extraction reaction was a proton-transfer process. Description of Extraction Equilibria. Equilibrium was governed by the mass action law;19 physical extraction of diluents was also taken into account. It was also under the following assumptions: the systems were diluent solutions and the activities of the interests and the complexes are proportional to their concentrations; the concentration of N235 in the organic phase is far higher than that of picolinic acid, and the complexes are mainly in the 1:1 form (N235/picolinic acid); reactive extraction and physical extraction accord with the simple addition; and the chemical reaction takes place at the phase interface. If N235 is defined as R3N and picolinic acid as HA, the following reaction equilibria exists. The two dissociation equilibria of picolinic acid: Ka1

H2A+ ←→ HA + H+ Ka2

HA ←→ A − + H+

/(10 pKa1− pH + 1 + 10 pH − pKa2)

K a2 =

[HA][H ] [H2A+]

= [R3N](KH[H+] + 1 + K1[HA])

B0 = [R3N](KH[H+] + 1)

⎛ ⎞ K1B0 ⎟ /(10 pKa1− pH + 1 vm + D=⎜ + ⎝ KH[H ] + 1 ⎠

(1)

[A −][H+] [HA]

(2)

+ 10 pH − pKa2)

KH

(3)

lg KH = − 0.7676B03 − 1.317B0 2 + 1.716B0

+

KH =

[R3NH ] [R3N][H+]

(11)

By fitting the experimental data, m was calculated to be 0.15. KH could be received from the apparent alkalinity, which was equivalent to lg KH. Figure 8 shows lg KH vs B0 according to a polynomial eq 12.

Equilibrium of N235 reacting with a proton:

R3N + H+ ←→ R3NH+

(10)

Substituting eqs 9 and 10 into eq 8, D was revised as

[HA] m= [HA]

HA ↔ HA

(9)

where B0 is the initial concentration of N235 (ignoring its dissolution in aqueous solution), pH is aqueous equilibrium pH, and v is volume fraction of diluent in the organic phase. Because the concentration of N235 was far higher than that of picolinic acid, namely, [R3NH+] + [R3N] ≫ ⎡⎣R3NH+A −⎤⎦, B0 could be rewritten as

Physical extraction equilibrium of diluents vs picolinic acid: m

(8)

B0 = [R3NH+] + [R3N] + [R3NH+A −]

+

K a1 =

(7)

+ 4.297

(4)

Chemical extraction equilibria of N235 reacting with picolinic acid:

(R2 = 0.9923, B0 = 0.04 − 0.85)

(12)

(6)

Then, K1 was calculated by eq 10 with experimental data (pHeq and D), and finally K2 was obtained from eq 6. Table 1 lists the fitting parameters of m, K1, K2, and KH. It was in the order of lg KH > lg K2 > lg K1, suggesting that under the equilibrium conditions involved in the present work, the trend of forming complexes tend to be through eqs 4 and 6 rather than eq 5. According to the parameters m, K1, K2, and KH, D (Dcal) were calculated. Comparing experimental D (Dexp) with Dcal, the fitting precision was examined. Figure 9 demonstrates the relationship of Dcal vs Dexp, which suggests the results are satisfactory.

where Ka1 and Ka2 are dissociation constants of picolinic acid with pKa1 = 2.21 and pKa2 = 5.29,20 respectively; m denotes the physical extraction equilibrium constant; KH represents the apparent reaction equilibrium constant of N235 with H+; and

CONCLUSIONS Extraction equilibria of picolinic acid between water and N235/ n-octanol were examined in detail. The D values were found to

K1

R3N + HA ↔ R3NH+A −

K1 =

[R3NH+A −] [R3N][HA]

(5)

K2 R3NH+ + A − ↔ R3NH+A −

K2 =

[R3NH+A −] [R3NH+][A −]

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Table 1. Model Parameters for Picolinic Acid + N235/nOctanol Systems v

m

lg KH

lg K1

lg K2

0 0.0422 0.1055 0.2109 0.3164 0.4218 0.6327 0.8436

1.00 0.98 0.95 0.90 0.85 0.80 0.70 0.60

0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15

4.367 4.462 4.593 4.684 4.729 4.661 4.347

2.104 2.021 1.838 1.760 1.734 1.562 1.321

3.027 2.849 2.535 2.366 2.295 2.191 2.264

Figure 9. Dcal vs Dexp.

be highly dependent on the aqueous equilibrium pH and the apparent alkalinity of N235/n-octanol. The optimum operation conditions were N235/n-octanol with the concentration of 0.4218 mol·L−1 at the equilibrium pH of 4.5−6.0. IR spectra analysis confirmed that equilibrium pH did not affect the structures of the complexes, and the ion-pair complexes were formed between N235 and picolinic acid, namely, the extraction reaction was a proton-transfer process. Both the reactions of N235 accepting a proton followed by reaction with anion picolinic acid and N235 directly reacting with neutral picolinic acid were the main processes, through which picolinic acid transferred into organic phases from aqueous phases. An extraction equilibrium model was established and fitted with experimental data.

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REFERENCES

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Figure 8. lg KH vs B0.

B0/mol·L−1

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Extraction Equilibria of Picolinic Acid with Trialkylamine - American

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